Voice-coil transducer

The moving mirror in most interferometers is driven by an electromagnetic transducer that operates on principles that are identical to the voice-coil transducer... 

Range of responses The highest and lowest frequencies that a microphone can successfully reproduce. Voice coU The bobbinless coil transducer element of a dynamic microphone. 

A force-feedback or active seismometer solves these problems by taking a mechanical system like that of the passive seismometer, as shown in Fig. 3, and adding a displacement transducer T, as shown in Fig. 5. In an active seismometer, the voice coil G is used to produce a force on the proof mass by driving it with a feedback current if instead of using it to produce a voltage proportional to the velocity of the proof mass. 

Whilst all DMAs utilise a voice coil type motor, some are fitted with a force transducer to measure the force transmitted through the sample. This requires a different design of instrument. It is frequently and incorrectly thought that the omission of a force transducer leads to poor modulus accuracy. Provided that the voice coil motor is correctly calibrated and its own contribution to the total stiffness measurement is removed, it does not make any difference, since the force measured by the transducer and the one taken from the motor calibration are both accurate to within better than 1%, which is adequate. Far larger errors occur... 

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